This invention relates generally to selectively etching polysilicon with respect to other materials including insulators such as oxide or nitride insulators.
In a variety of different circumstances, it may be desirable to etch polysilicon selectively. That is, it may be desirable to preferentially etch polysilicon while reducing the etching of other materials such as insulators.
One example of a situation where such selectivity may be desirable is in connection with providing dual metal gate technology. Dual polysilicon gates are used in conventional complementary metal oxide semiconductor (CMOS) devices to engineer a desired threshold voltage that may be different between the NMOS and PMOS devices. Unfortunately, as the device""s scale becomes smaller, this approach is not effective. When the polysilicon doping level is not sufficiently high, the polysilicon gate depletion effectively increases the gate dielectric thickness by several Angstroms. This negatively impacts the ability to scale gate dielectric thicknesses. Boron penetration and gate resistance may also be issues for such polysilicon gate technology.
One approach to this problem is to replace the polysilicon gate with a metal gate. More particularly, one metal gate may be utilized for the NMOS devices and a different metal gate may be utilized for the PMOS devices.
Thus, it may be desirable to form dual metal gate technology from conventional processing steps that use polysilicon. After the polysilicon has been defined to form the gate electrodes for a transistor, the polysilicon may be selectively removed. A different metal may be applied to form each of the NMOS and PMOS transistors.
Thus, there is a need for a better way to selectively etch polysilicon with respect to other materials.